This invention relates to an electronic circuit device comprising a photoimageable covercoat comprising an epoxy-modified aromatic vinyl-conjugated diene block copolymer
A major trend in the electronics industry is to make products smaller, lighter and faster while maintaining or improving their functionality. One key technology enabling the creation of more compact products involves electronic packaging and assembly. Electronic packaging and assembly constitutes the materials and processes required to interconnect a semiconductor chip (IC chip) to other electronic or electrical components. Besides the semiconductor chip, various materials can be incorporated into an electronic package, such as flexible circuitry (metal circuitry on polyimide or other polymer films), metal stiffeners, electrically conductive layers, electrically insulating layers, and heat sinks.
Flexible circuitry is commonly found in applications ranging from medical devices to hard disk drives to inkjet printer pens. Flexible circuits, thus used, can be exposed to a wide range of environmental factors that may cause degradation in the circuitry over time. Some of the harsh environments that a flexible circuit might see during its lifetime include high temperature and/or high humidity, cycling between temperature extremes, submersion in corrosive fluids during which the circuit has an applied voltage bias across it. If the circuit is not sufficiently protected during this exposure, damage may occur including a decrease in electrical performance, corrosion of circuit traces or even device failure.
The circuit traces are commonly protected from temperature and other environmental factors, including solvents, inks, and other corrosive chemicals by applying a durable insulating material on the exposed surface of the circuitry. This insulating layer is frequently referred to as a xe2x80x9ccovercoatxe2x80x9d or a solder mask. Covercoats and encapsulants are used to protect the circuitry, and are applied by a variety of precise placement printing processes, such as screen printing, transfer printing and the like, to ensure that sections of circuitry which need to make further connections will be left uncoated. However, as the pitch of the circuitry continues to shrink, current application methods become less capable of meeting the fine pitch placement requirements. One possibility for overcoming this problem would be to use a photoimageable covercoat that can withstand the environment to which the package may be subjected.
The use of photoimageable materials in flexible circuit manufacture as photoresists is known. In U.S. Pat. No. 5,227,008, use of a dry film photoresist is disclosed, and in U.S. Pat. No. 6,177, 357, a liquid photoresist is disclosed. However, such photoresists are designed to be temporary in nature, rather than providing long term protection. They are completely removed during the flexible circuit manufacturing process.
A photoimageable polyimide is disclosed in U.S. Ser. No. 09/547,390, which is deposited by electrophoretic deposition using aqueous acid developers.
Epoxidized styrene-diene block copolymers, such as epoxidized styrene-butadiene or epoxidized styrene-isoprene block copolymers, have been described in U.S. Pat. No. 5,478,885. In some applications, the epoxidized block copolymers have been used as rubber toughening agents in commonly-used epoxy resins. Typically, the toughening agent constitutes a small percentage of the total composition.
Cured compositions comprising epoxy resins and epoxy-modified aromatic vinyl-conjugated diene block copolymers have been described in EP 658603. The compositions can contain from 5 to 95 parts by weight of an epoxy resin, preferably from 20 to 80 parts by weight. When the composition contains less than 5 parts by weight of either component, a loss of mechanical properties is reported. Compositions comprising cured epoxidized styrene-diene block copolymers useful as multilayered molding materials have been described in WO 98/22531. Compositions further comprising from 0.01 to 25 parts by weight added polyfunctional co-reactants are also described. Epoxy resins are not disclosed as co-reactants. Use of these materials as covercoats in electronic packaging applications is not suggested.
Briefly, this invention provides an electronic circuit device comprising a photoimageable covercoat composition including at least about 95 weight percent of a curable epoxy-modified aromatic vinyl-conjugated diene block copolymer, up to 5 weight percent of an epoxy resin, such weights being exclusive of catalyst, and from about 0.25 percent by weight to about 1.5 percent by weight of an epoxy catalyst.
In one embodiment, the catalyst is present as a catalyst solution; in an alternate embodiment, the catalyst is present in solid form.
In another aspect, this invention provides a method of using a curable resin composition in an article comprising the steps of:
forming a curable resin composition on a liner by a means selected from the group consisting of extrusion and solvent coating, wherein the resin composition includes in the range of 95 to 100 weight percent of an epoxy-modified aromatic vinyl-conjugated diene block copolymer, optionally up to 5 weight percent of an epoxy resin, the weight of both components based on the weight of epoxy-bearing materials, and from about 0.25 to about 1.5 weight percent of an epoxy catalyst, and laminating the covercoat onto at least a portion into an electronic circuit device.
Upon photoimaging and curing the copolymer resin composition, it exhibits good contrast, and low swelling in the areas exposed to ultraviolet radiation, while using low photoinitiator concentrations. The cured copolymer resin composition also provides a protective permanent covercoat for the electronic circuit.
In this application, these terms have the following meanings.
1. The term xe2x80x9celectronic circuit devicexe2x80x9d means a device comprising an electronic circuit or electronic component, such as (1) an electronic package such as a ball grid array (BGA), a laminated microinterconnect (LMI), multi-chip module, or a chip scale package (CSP); (2) simple flexible circuitry wherein a copper foil is adhered to a polymer substrate with an adhesive, or plated onto a polymer substrate either selectively or totally; or (3) an electronic component such as a semiconductor chip connected to a circuit on a substrate.
2. The term xe2x80x9celectronic circuitxe2x80x9d means the path of an electric current or electrons and can include such elements as electric conductors, e.g., metallic wires or metallic traces, and electronic components such as semiconductor chips, transistors, diodes, capacitors, resistors, inductors, etc.
3. The term xe2x80x9cepoxy-bearing materialxe2x80x9d means an epoxy-modified aromatic vinyl-conjugated diene plus epoxy resin, if present.
4. The term xe2x80x9csolder resistancexe2x80x9d means resistance to heat at the temperature of solder reflow.
5. The term xe2x80x9cresin compositionxe2x80x9d means an epoxy-bearing material plus a curative or catalyst.
6. The term xe2x80x9cepoxy-modified aromatic vinyl-conjugated diene block copolymerxe2x80x9d includes such copolymers that are partially hydrogenated prior to or subsequent to epoxy-modification.
7. The term xe2x80x9ccatalyst solutionxe2x80x9d means a 50% by weight of a triarylsulfonium hexafluoroantimonate salt mixture in propylene carbonate, such as that available as UVI-6974, from Dow Chemical Company, Midland, Mich.
8. The term xe2x80x9ccontrastxe2x80x9d means a measure of the material response between ultraviolet radiation exposed and nonexposed regions during the developing process. High contrast would provide for removal of unexposed regions of the material without any undesirable impact on the exposed regions. Low contrast would suggest that removal of unexposed regions leads to damage such as erosion or swelling of the exposed regions.
All amounts, ratios and ingredients herein are by weight unless otherwise specifically noted.
The copolymer resin composition has shown no ink sorption by visual observation after immersion for eleven days at room temperature. The copolymer resin composition also has excellent resistance to corrosive, aqueous acidic and/or alkaline environments.